Study of lead phytoavailability for atmospheric industrial micronic and sub-micronic particles in relation with lead speciation

Abstract

Particles from channelled emissions of a battery recycling facility were size-segregated and investigated to correlate their speciation and morphology with their transfer towards lettuce. Microculture experiments carried out with various calcareous soils spiked with micronic and sub-micronic particles (1650 ± 20 mg Pb kg⁻¹) highlighted a greater transfer in soils mixed with the finest particles. According to XRD and Raman spectroscopy results, the two fractions presented differences in the amount of minor lead compounds like carbonates, but their speciation was quite similar, in decreasing order of abundance: PbS, PbSO₄, PbSO₄·PbO, α-PbO and Pb⁰. Morphology investigations revealed that PM_2.5 (i.e. Particulate Matter 2.5 composed of particles suspended in air with aerodynamic diameters of 2.5 μm or less) contained many Pb nanoballs and nanocrystals which could influence lead availability. The soil–plant transfer of lead was mainly influenced by size and was very well estimated by 0.01 M CaCl₂ extraction.

Introduction

Due to its numerous past and present uses and high persistence, lead is a major environmental contaminant (Chen et al., 2005). Potentially toxic for living organisms even at low concentrations, lead constitutes a risk for humans who can absorb it in various ways (Canfield et al., 2003). In the context of contaminated gardens, elevated lead intake by humans can be due to the consumption of crop plants grown on soils with relatively high plant-available metal concentrations, ingestion of contaminated soil, either accidentally or intentionally (pica), inhalation of soil particles and drinking water with high soluble concentrations of metals (Alexander et al., 2006). The total quantities of lead emitted in the environment by industries have decreased sharply in recent decades (Glorennec et al., 2007) and are strictly controlled in Europe nowadays. Lead was recently classified as a substance of very high concern in the European REACH law (Regulation EC 1907/2006 of the European Parliament and of the Council of 18 December 2006 concerning the Registration, Evaluation, Authorisation and Restriction of Chemicals). However, particles enriched with lead are still generated especially by lead-recycling plants (Batonneau et al., 2004, Ohmsen, 2001) and constitute the main source of lead pollution for soils (Miquel, 2001, Donisa et al., 2000).

According to Zhang et al. (2005), emitted particles present a large granulometric spectrum in the atmosphere, but during the last decade the proportion of fine particle matter (PM) increased with the use of more effective filters in industry. Indeed PM₁₀ are target species of the World Heath Organization (WHO, 1987) and the European Union Framework Directive on ambient air quality assessment (European Commission, 1999), due to their adverse effects on the environment and human health. While micrometric and sub-micrometric fractions contribute very little to ambient particle mass, they may occur in substantial number concentrations. Most of the studies dealing with the characterization of metal-enriched particles in the ambient air provide information on quantitative measurements for PM₁₀ fractions (EU directives 96/62 and 99/30) and very few on the sub-micronic fraction (Lazaridis et al., 2002). The lack of knowledge regarding metal speciation in the industrial particles results mainly from a lack of analytical tools, both sensitive and specific to the size of the particles.

These fine particles are highly reactive due to their high specific area and can be transported over long distances in the troposphere (Barrie, 1992). They could therefore present a greater impact on the biosphere than coarse particles (Fernandez Espinosa and Rossini Oliva, 2006). Ruby et al. (1992) concluded that the bioaccessibility of lead rises strongly in particles under 2.4 μm size. But, the phytoavailability of lead in industrial particles as a function of their size and speciation has not been studied yet. In comparison with zinc, lead generally shows a relatively low mobility in soils (Dumat et al., 2006). It can however migrate through the soil with dissolved organic matter (Cecchi et al., 2008) or be mobilized by certain plants (Arshad et al., 2008). Moreover, carried from the air to the soils as fine particles, lead could be released more easily in soil solution (Komarnicki, 2005).

We therefore focused our study on the links between soil–plant transfer of lead, size and speciation of particles emitted by a lead-recycling plant, currently the main source of atmospheric emissions (Cecchi et al., 2008). The objectives were the following: (i) the elemental and molecular characterization of micrometer and nanometer sized lead-rich particles and (ii) to study the influence of particle characteristics on lead soil–plant transfer.

The physico-chemical characterization of industrial PM₁₀ and PM_2.5 particles collected in the plant was investigated using both bulk and microanalysis techniques: (i) MEB-EDS to determine the morphology and chemistry on the scale of a particle (Laskin et al., 2006, Choël et al., 2005, Choël et al., 2006); (ii) Raman microspectrometry to study particle speciation (Batonneau et al., 2004, Batonneau et al., 2006, Falgayrac et al., 2006, Sobanska et al., 2006). The transfer of lead from particles to the lettuce, Lactuca sativa, a widely grown garden vegetable was investigated in the laboratory: two different uncontaminated calcareous soils were spiked with PM₁₀ and PM_2.5 for soil–plant experiments with a biotest device that enabled careful study of rhizosphere and roots in addition to the transfer to the shoots (Chaignon and Hinsinger, 2003). The study was finally completed by CaCl₂ extraction experiments carried out according to Houba et al. (1996) to estimate lead phytoavailability.

The hypothesis tested throughout all these experiments was that particle characteristics have a significant influence on lead soil–plant transfer and translocation.